The wheel tractor scraper is a machine employed in various industries, such as agriculture, construction and mining to load, haul, eject and spread layers of earth. Such machines are particularly suited for applications, for example, in roadway construction and site preparation, where material needs to be removed or added while creating or maintaining grade and hauling occurs over moderate distances, e.g. under one mile. Conventional wheel tractor scrapers typically include a tractor portion having a forward frame member that supports the operator station and a power source operatively coupled to the driven wheels of the machine. An articulated joint couples the tractor portion to the rear scraper portion, the scraper portion having a rear frame member that supports both a bowl for collecting and hauling material, and the rear wheels. During operation, the bowl is typically lowered to engage the ground along a cutting edge that is driven forward by the machine, loading the bowl. Many of these machines will have an earth-moving work tool, such as an elevator, conveyor, auger, or spade, associated with the bowl to facilitate penetration and/or loading of the material to be transported.
One of the limiting factors associated with wheel tractor scraper operations are the traction conditions of the work site. Tractor scraper operations can be limited, for example, by the type of material, geographic location, and seasonal conditions of the work site.
Various improvements and methods of operation have been adopted by the industry to increase the versatility and efficiency of these machines. For example, wheel tractor scrapers are often employed in push-pull operations, wherein a first tractor scraper is either pulled or pushed by a second machine, for example, a track-type dozer or another wheel tractor scraper, during the loading process. Wheel tractor scrapers are often provided with hitches or push bars to facilitate these operations. However, the option of a second machine is not always possible, and this increases operating costs. Further, this does not address concerns of the tractor scraper becoming stuck during the remainder of the work cycle.
As an alternative, some large wheel tractor scrapers are provided with an additional, rear mounted engine operatively connected to drive the rear wheels of the machine (twin-engine scrapers), making these machines better suited for handling adverse terrain and worksite conditions. However, another alternative has been to provide a fluid operated rear wheel assist.
For example, U.S. Pat. No. 5,682,958 to Kalhorn et al. provides a hydrostatic rear wheel assist that includes a reversible variable displacement pump operatively coupled to an engine and mounted to the front frame section of an articulated scraper. The pump is fluidly connected to a pair of motors positioned on the rear frame section for driving the right and left rear wheels, respectively. The pump may be actuated via a floor pedal that controls an engagement/disengagement valve having two positions, an engagement position for directing pressurized fluid to the motors, and a disengagement position for preventing flow to the motors. However, this requires an additional and dedicated fluid pump, fluid lines, and other components that significantly add to overall vehicle complexity and cost.
Another difficulty associated with providing a rear wheel assist for an articulated machine is that as the articulation angle is increased to effectuate a turn, if too much power is supplied to the rear wheels, and the traction of the front driven wheels is insufficient, the machine may be driven forward rather than turning. This may also cause the front end of the machine to “hop” when the front wheels catch or regain traction. The result of both of these conditions is decreased machine control and undesirable stresses that may damage the machine.
In general, the need exists in the industry for wheel tractor scrapers that are capable of efficient operation under a greater range of terrain conditions. In particular, the need exists for an improved rear wheel assist design and efficient methods of operation thereof, and, more particularly, for a rear wheel assist that responds to machine articulation.